Imaging and/or raster-mode scanning system provided with a device for compensating the image degradations resulting from environmental factors

ABSTRACT

An imaging and raster-mode scanning apparatus has a compensation device for compensating for ambient influences that may degrade the imaging, comprising an electrical filter, and at least one sensor for providing a first signal dependent on the ambient influences the first signal passes through the filter directly and drives an internal actuator and a internal control elements of the apparatus, which has an effect on the imaging and on the image display, in a calibrated state of the apparatus, which comprises a setting of a transfer characteristic of the filter, image degradations are greatly reduced or essentially compensated for. The filter for calibrating the apparatus, has a calibration input and a second signal is applied to the calibration input of the filter.

CROSS-REFERENCES TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

The invention relates to an imaging and/or raster-mode scanningapparatus and to a method for operating an apparatus of this type with adevice for compensating for ambient influences that may cause imagingdegradations.

FIELD OF THE INVENTION

Imaging and/or raster-mode scanning apparatuses, for example scanningelectron microscopes, force microscopes and light scanning microscopes,have attained great importance in many methods for inspecting samples.

However, these measurements are frequently influenced by externalambient conditions such that the imaging quality is diminished. Thisresults, under certain circumstances, in an undesirable degradation ofthe resolving power and/or in defective imaging. In the following text,an imaging degradation of this type is generally described as theoccurrence of imaging or image defects. In the case of electron scanningmicroscopes, by way of example, an influencing variable that diminishesthe imaging quality may be an electromagnetic interference field whichinfluences the electron orbits.

Furthermore, air and/or ground vibrations in the surroundings of themicroscope are a factor for consideration, these causing losses ofspatial definition in the illumination of the sample and/or in thedetection of the electrons. The above-described influence ofelectromagnetic interference fields or air and/or ground vibrations onthe imaging quality applies, in principle, to, all imaging and/orraster-mode scanning apparatuses.

One method for eliminating air and/or ground vibrations consists forexample in putting the apparatus onto a vibration-damping orvibration-insulating apparatus. However, devices of this type are veryexpensive. Moreover, these devices offer only limited protection againstthe abovementioned ambient influences, above all at very lowinterference frequencies, as may occur in the event of buildingvibrations, for example.

In the case of electromagnetic and/or magnetic interference fields,according to the prior art, these fields are detected and compensatedfor by means of inducing a current flow through a coil outside theapparatus. This method has the disadvantage that although theinterference fields are significantly reduced, by means of negativefeedback, at the location where the interfering quantity is detected,this is not necessarily the case at the “actual location of occurrence”,that is to say along the electron orbits in the case of an electronscanning microscope.

SUMMARY OF THE INVENTION

The object of the invention, therefore, is to provide an apparatus inwhich ambient influences that may cause imaging degradations or defectsare compensated for effectively and without a high financial outlay.

This is achieved in a surprisingly simple manner by means of anapparatus according to the invention and a method for operating anapparatus of this type according to the invention.

Accordingly, a first signal dependent on the ambient influences passesthrough an adjustable digital electrical filter and drives an actuatorand/or a control element which has an effect on the imaging and/or onthe image display, in which case, in the calibrated state of theapparatus, which is realized by setting the transfer parameters, that isto say the transfer characteristic of the filter, the image degradationis greatly reduced or essentially compensated for. Setting the filtermakes it possible to ensure that the compensation of the ambientinfluences interfering with the imaging takes place “at the actuallocation of occurrence”, in contrast to apparatuses according to theprior art. The invention can be utilized in a multiplicity ofembodiments. In these cases, the outlay can be made dependent on therequired degree of compensation of the ambient influences. By way ofexample, the digital filter, for calibrating the apparatus, may have acalibration input to which a second signal for setting the transferparameters of the filter is applied, or the filter may have a device formanually setting the transfer parameters. If an output signal of theimage processing device is applied to the calibration input of thefilter, then, in dependence on the image defects detected, the transferparameters of the filter can be coordinated in such a way that theinterference compensated for is exactly that which affects the imaging,and not the interference at a location in the vicinity of the apparatus.

The first signal which is dependent on the ambient influences and isapplied to the signal input of the filter can either be output by asensor for detecting at least one physical quantity outside theapparatus, or an output of the image processing device is connected tothe calibration input of the filter, with the result that thecalibration signal depends on an image analysis, for example. If asensor is used to output the first signal, it is possible to detectelectromagnetic and/or magnetic fields, air vibrations and/or body orground vibrations. In an advantageous manner, an interfering quantity,or alternatively more than one simultaneously, can be picked up and theimaging defects caused by the interfering quantity can be compensatedfor by the driving of one or more control elements.

The high flexibility of the invention is also demonstrated in the factthat the effect according to the invention on the imaging and/or on theimage display can take place in dependence on the interfering quantitiesin diverse ways. The actuators and control elements used may preferablybe internal ones that are present, for example deflection systems oradjustment arrangements of sample stages. In addition to actuators whichare assigned to the scanning device, it is possible, furthermore, touse, as further actuators, all systems which, like force actuators ordistance drives, are sensitive to vibration, for the purpose of applyingthe correction signal. Furthermore, it is also possible to realize thecompensation of the imaging defects by driving a control element in animage processing device, without influencing the defective imagingitself. In this case, this control element in the image processingdirection comprises for example an adjustable parameter for acalculation in the image processing device. The use of multi-axissensors and control elements advantageously enables the compensation ofinterference in a number of spatial directions. For this purpose, it ispossible, by way of example, to vary the calibration signal at thefilter as a function of the scanning location and/or of time.

In an embodiment, the apparatus, for example a microscope, is operatedin a calibration mode and subsequently in an image mode, whereby, in thecalibration mode, ambient influences that degrade the imaging aredetected by the imaging of a predetermined reference object andcomparison of the image with the real structure of the reference object,and are greatly reduced or essentially compensated for by calibration,and whereby the imaging defects are compensated for by maintaining thecalibration in the imaging mode, even in the event of a change in theambient influences.

By virtue of the comparison of the image with the real structure of areference object, the compensation of the interfering ambient influencesis carried out on the basis of the imaging defect that is objectivelypresent. As a result, furthermore, in addition to the ambientinfluences, systematic imaging defects of the apparatus can also bedetected and eliminated. While minor fluctuations in the interferingquantity are automatically compensated for, greatly altered ambientconditions, for example caused by the microscope being sited in a newplace, can easily be taken into account by means of a calibration cyclein which a new calibration, adapted to the altered conditions, of theapparatus is carried out. The apparatus can be calibrated anew atpredetermined time intervals, whereby even changes in the ambientconditions which are not obvious are automatically taken intoconsideration.

The calibration mode is distinguished by the fact that a correlation isproduced between the respective imaging defects that have been detectedand the interfering influence detected by a sensor.

Conversely, this means that, from an interfering influence detected by asensor outside the apparatus, a conclusion can be drawn about theimaging defect caused by this interfering influence and this imagingdefect can be compensated for. Moreover, by means of external driving ofthe scanning device of the apparatus, it is possible to detect aselected section of the reference object, for example along a circle,repeatedly at time intervals. In this way, time-variable imagingdefects, for example caused by a building vibration, are alsoidentified. By varying the scanning distance, for example by alteringthe scanning radius, it is possible, moreover, to detectlocation-dependent imaging defects, that is to say imaging defects whichdepend on the scanning location of the exemplary scanning microscope.Consequently, the apparatus according to the invention is set up for thedetection and compensation of location- and time-dependent imagingdefects.

In the image mode, the actual sample is then detected in its entirety byscanning, the second signal, for setting the transfer characteristic ofthe filter, advantageously being derived using the data determinedduring the calibration mode as a basis.

In a further embodiment, the apparatus is set up for automaticallycalibrating the filter during the image mode. In contrast to thepreceding embodiment, the calibration is carried out during the normalimage mode. Consequently, by way of example, the customary microscopicsequence is not disrupted since it is not necessary to carry out achangeover between a sample and the reference object. In addition to theadvantage of requiring less time, the apparatus responds directly towhat may be an unnoticed change in the interfering quantity and iscalibrated anew by the transfer characteristic of the filter being set,the signal applied to the calibration input of the filter being derivedfrom an image analysis in the image processing device. By means of aline-by-line image analysis, the displacement of the line centroids ofsuccessive image lines within the whole image can be determined, forexample recursively, and a second signal can be calculated from thistemporal displacement for the purpose of driving the calibration inputof the filter. The pixel displacements of the line centroid thus serveas the amplitude of the image interference. The line frequency permitsan assignment of time and frequency for a correlation consideration inthe case of the active application of a compensation signal dependent onthe interfering quantity, that is to say in the case of the driving ofan actuator and/or of a control element which have an effect on theimaging and/or the image display. If a sensor arranged outside theapparatus and serving to detect an ambient influence which degrades theimaging is read in in parallel with the interference amplitudedetermined, at the start of each line, then this enables thesimultaneous pick-up of image interference and the external interferinginfluence causing the latter. This method thus permits a directcalculation of the transfer function of the filter, which is required inorder to compensate for the interference. As an alternative, fundamentalassumptions may be made, for example with regard to the number of polesand zeros of the transfer functions, and individual parameters, that isto say, for example, the poles and zeros, can be optimized iterativelyby means of the image analysis. The line-by-line image analysis permitsthe filter to be set and thus the ambient influences causing the imagingdefects to be compensated for, up to a frequency corresponding to halfthe detection frequency, in accordance with the Nyquist theorem.

The image analysis may also comprise the recursive determination of thedisplacement of the image centroid of successive images. This isappropriate for example for transmission electron microscopes or lightmicroscopes, which use a camera system for displaying an object. Bydetermining the displacement of the image centroid in two mutuallyorthogonal axes, it is thus possible, by means of a correspondingcorrelation with the interference quantities, to rectify the imagedefects in two mutually perpendicular directions by the driving ofcorresponding actuators and/or control elements. The camera systemsdiscussed conventionally operate between 25 and 70 Hz. Although theevaluation and thus also the compensation by the application ofcompensation quantities even at frequencies which are higher than theimage frequency of the camera system used.

In a further advantageous embodiment of the invention, not only thecalibration input of the filter is fed by the image processing device,but also the signal input of the filter. Consequently, it is possiblefor the forward-connected sensor to be dispensed with and only thedisplacements, obtained from the image analysis, to be fed back intosuitable control elements/actuators in two mutually orthogonaldirections, in which case the said control elements/actuators, as in allthe previous advantageous embodiments, may be assigned to the scanningdevice and/or to the image processing device or alternatively may befurther actuators.

The invention can be used in a multiplicity of imaging and/orraster-mode scanning apparatuses which are suitable for the productionor observation and measurement of surfaces, for example scanningelectron microscopes, force microscopes, surface roughness measuringinstruments, optical scanning microscopes, light microscopes,transmission electron microscopes or lithography installations.

Existing installations can be equipped by simple retrofitting to giveapparatuses according to the invention for compensating for ambientinfluences.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described below on the basis of a number of embodimentswith reference to the appended drawings, in which:

FIGS. 1 a to 1 d show different embodiments of the invention in the formof block diagrams,

FIG. 2 schematically illustrates a scanning microscope according to theinvention,

FIG. 3 illustrates an exemplary reference object, of the kind that canbe used for the calibration mode of the microscope in FIG. 2,

FIG. 4 shows an exemplary signal S of the image acquisition device whenthe microscope in FIG. 2, in the calibration mode, scans and acquires areference object on a predetermined path 9 in accordance with thecoordinate x at different times,

FIG. 5 shows the exemplary correlation between the displacement of theline centroids, which is illustrated by the curve 15, and the temporallycorresponding profile 14 of an interfering quantity which is detectedoutside the apparatus and causes the displacement of the line centroids,

FIGS. 6 a to 6 c show the displacement of the image centroid of threesuccessive images,

FIG. 7 shows the temporal profile 17 of the displacement of the centroidfrom FIG. 6 for the x-direction, and

FIG. 8 shows an embodiment of an optical microscope corresponding to theblock diagram of FIG. 1 c.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 a schematically illustrates an embodiment of the imaging and/orraster-mode scanning apparatus according to the invention in the form ofa scanning electron microscope in a block diagram. The numeral 1designates an-apparatus for scanning sample objects by an electron beam(see also FIG. 2) however not including an internal actuator 3 such asdeflection coils 3 a, 3 b, an image processing device 2 and acompensation device for compensating for ambient influences which maydegrade the imaging. The term “scanning electron microscope” alsoincludes device elements such as the image processing device 2, theinternal actuator 3, and other elements necessary for operating themicroscope. There is a sensor 4 outside the microscope, this sensor 4outputting a first signal which is dependent on ambient interferinginfluences U, for example an electromagnetic interference field at thelocation of the sensor 4. A-digital filter 5 has a transfercharacteristic which can be set at a calibration input manually (FIG. 1a) or automatically (FIGS. 1 b to 1 d). The interfering influence Uaffects both the sensor 4 and the apparatus 1, this being indicated, inFIGS. 1 a to 1 d, by the arrows proceeding from U. The first signal isprocessed in the filter and amplified in a regulating amplifier 6,connected in a loop to the internal actuator 3 such as the electron beamdeflection coils 3 a, 3 b of the scanning electron microscope. The firstsignal is a compensating signal dependent on the ambient influences andis applied to the signal input of the filter 5. In the embodiments ofFIGS. 1 a to 1 c the sensor 4 is for detecting at least one physicalquantity outside the microscope, and in the FIG. 1 d embodiment, theimage processing device 2 produces the first compensating signal. Theregulating amplifier 6 serves for matching the output signal of thefilter to the internal actuator 3. Thus, the first compensation signalwhich is dependent on the interfering quantity, here the electromagneticinterference field, is combined with an actual driving signal of thedeflection coils 3 a, 3 b. It should be understood that the sensor 4 isarranged outside the microscope in such a way that the driving of theactuator 3 does not significantly influence the first signal. The effectachieved by the calibration of the filter 5 is that the applied firstcompensating signal for the image acquisition corresponds precisely toan opposite effect which is caused by the interfering influence U, herethe electromagnetic interference field at the location of the apparatus1. Consequently, the effect of the first compensating signal and theeffect of the interfering electromagnetic field on the imagingessentially cancel each other out. If the scanning electron niicroscpeis moved to a different site, the filter must be calibrated for thepurpose of modeling the transfer function.

FIG. 1 b shows a block diagram of an arrangement according to theinvention, in which the calibration of the filter 5 is carried outautomatically by means of a second signal from an image processingdevice.

FIG. 2 shows a scanning electron microscope including an imageacquisition device which acquires at least one pixel of an object to bescanned and supplies corresponding signals to an image processing device2. The image processing device 2 also generates calibrating signals andis connected to the calibration input of the filter so as to calibratesame. As in the case of the first embodiment, the first compensatingsignal of the sensor 4 is fed forwards through the filter 5 and theamplifier 6 to internal actuator implemented by the deflection coils 3a, 3 b. The calibration of the filter 5 and thus of the apparatus isdescribed below with reference to two different embodiments.

According to a first embodiment, the microscope is set up for operationin a calibration mode and an image mode, whereby, in the calibrationmode, ambient influences that reduce the imaging quality can be detectedby the imaging of a predetermined reference object, such as shown at 8in FIG. 3, and by comparing the image with somewhat corresponding to“the real structure” of the reference object. The ambient influences canessentially eliminated by calibration of the microscope, i.e. theimaging defects are greatly reduced or essentially compensated for, evenin the event of a change in the ambient influences, by maintaining thecalibration in the image mode. Depending on the operating mode, theinput signal of the calibration input of the filter 5 either depends onthe respective measured imaging defect (calibration mode) or is obtainedby means of the data stored during the calibration mode (image mode). Itis possible to switch back and forth between the calibration and imagemodes.

The calibration mode is utilized in order to detect ambient influences,that is to say in this case the electromagnetic interference field whichreduces the imaging quality, by the imaging of a predetermined sectionof the reference object 8 and comparison of such image with “the realstructure” of the reference object, and to calibrate the apparatus bysetting the transfer characteristic of the filter in such a way thatsystematic imaging defects caused by external ambient conditions and/orcaused by instrumentation are essentially compensated for.

To this end, an undistorted image of the reference object 8 (FIG. 3) hasbeen stored previously in the image processing device 2.

The scanning microscope acquires an actual image of the reference object8 by scanning a selected section of the reference object 8, for examplealong circle 9 as shown. This previously stored image of the referenceobject 8 is compared with the actual image of the reference object asobtained from the image acquisition device, and a signal assigned to thedifference between previous and actual image is formed and is output tothe calibration input of the filter.

The calibration method in the calibration mode can be described by thefollowing steps:

-   -   determining a first signal, which depends on the electromagnetic        interference field at the location of the sensor, by a sensor 4;    -   applying the first signal to the signal input of the filter 5;    -   acquiring a selected section of a predetermined reference object        by means of an image acquisition device 7 by scanning the        reference object;    -   comparing the acquired image with an undistorted image of the        reference object;    -   determining an error signal assigned to the difference;    -   applying the second signal, derived from the error signal, to        the calibration input of the filter for resetting the transfer        characteristic of the filter;    -   applying the output signal of the filter 5 to the signal input        of the regulating amplifier 6;    -   applying the output signal of the regulating amplifier 6 to the        electron beam deflection coils for the purpose of correcting the        reduced image quality;    -   iterative calibration of the characteristic of the filter 5, in        such a way that the reduction of the imaging quality is greatly        reduced or essentially compensated for, by means of the        following steps:    -   comparing the corrected image with an undistorted image of the        reference object;    -   modifying the transfer characteristic of the filter 5 so as to        approximate the corrected image to the undistorted image of the        reference object;    -   storing data for generating the determined transfer function of        the filter 5 for the image mode.

In one embodiment, these data for generating the determined transferfunction of the filter 5 for the image mode are stored in a memoryassigned to the image processing device 2. In a further embodiment, thefilter 5 is set up for storing these data. While the imaging defect isbeing determined, the devices for compensating for the imaging defectsare switched off. The microscope according to the invention is thencalibrated by the method described above, that is to say the feedforward for the measurement signal of the sensor is set as a measure ofthe interfering quantity.

The compensation quality is measured by repeated scanning of thereference object and comparison of the image with the real structure ofthe reference object. By determining the compensation quality in eachcase and correspondingly changing the transfer function of the filter,the feed forward is iteratively changed in such a way that the imagingdefects of the scanning electron microscope are minimized.

The microscope can be calibrated with regard to location- and/ortime-variable imaging defects.

For this purpose, a reference object as shown in an exemplary fashion inFIG. 3 is scanned on a predetermined path in the calibration mode. Theimaged reference object comprises circular gold deposits which have beendeposited on a substrate and are arranged in a predetermined manner. Thescanning device of the microscope is driven externally, with the resultthat a selected section of the reference object is acquired. This pathmay, for example, be closed like that shown by the curve 9. Individualobjects 8 are situated on this closed path, to which objects the imageacquisition device 2 responds and generates a signal not equal to zero.This is shown schematically and by way of example in FIG. 4, whichillustrates the signal profile 10 acquired at a given instant t_(i)during the traversal of the closed curve 9. Time-dependent interferencecan cause time-dependent imaging defects. For this reason, in theillustration of FIG. 4, the closed curve has been traversed four timesat intervals. The resulting four signal profiles 10 are thus also ameasure of the temporal dependence of the interference. Furthermore, thetraversed curve is altered by varying the radius R, wherebylocation-dependent imaging defects can be detected. According to theinvention, the time- and/or location-dependent imaging defects aredetermined by comparison of the image in the image processing device 2with the predetermined reference object, which is known exactly. In theexample represented in FIG. 4, the time-dependent imaging defect isillustrated by the curve 11.

The image mode is the operating mode of the inventive scanning electronmicroscope in which the actual sample is measured. The filter transfercharacteristic determined in the calibration mode is invariant duringthe subsequent image mode with regard to the characteristic defined inthe calibration mode. As explained above, however, it can vary withrespect to time and as a function of the scanning location.

Assuming an essentially constant correlation between the electromagneticinterference field and the imaging defect caused by this interferingquantity, the output signal of the filter 5, after passing through theregulating amplifier 6, is applied to the electron beam deflection unit3, with the result that image defects are essentially compensated foreven in the event of a change in the ambient influences, that is to saythe strength of the electromagnetic interference field.

In an embodiment developed further, in addition to the electromagneticinterference fields, air vibrations and/or ground vibrations are alsodetected by corresponding sensors, the signals that are output passthrough calibratable filters which are assigned to the respectiveinstances of interference and have adjustable transfer characteristics,and, after additional matching in the regulating amplifier 6, areapplied to the deflection unit as a further control signal and/or toother actuators, with the result that the imaging defects caused by airvibrations and/or ground vibrations are also greatly reduced oressentially compensated for.

The necessity of having to switch back and forth between differentoperating modes of the apparatus is overcome in the embodiment describedbelow by virtue of the fact that the apparatus is set up for automaticcalibration of the filter during the image mode. In order to simplifythe explanation, this embodiment is again described with regard to ascanning electron microscope, but is not restricted thereto. Theapparatus essentially comprises the components of the scanning electronmicroscope described above, with the exception that in the imageprocessing device the acquired image is analysed and a signal dependenton the analysis is applied as second signal to the calibration input ofthe filter.

In the exemplary embodiment, this image analysis comprises the recursivedetermination of the displacement of the line centroids of successiveimage lines within the whole image. The analysis is based on the insightthat images of objects in imaging and/or scanning apparatuses aregenerally not stable with respect to time on account of the influence ofthe interfering quantities of the imaging. For elucidation purposes,FIG. 5 illustrates the profile of the brightness within four selectedimage lines, the centroids of the brightness distribution in each linebeing identified by a circle and the curve 15 illustrating thedisplacement of this centroid of the chronologically successivelyscanned lines. In a manner corresponding to the respective lineacquisition instants, the magnitude of an exemplary interfering quantitywhich causes the corresponding pixel displacement of the centroid withinthe lines is plotted as curve 14 on the left-hand side. In this way, itis possible to produce a correlation between the interfering quantityand the imaging defect caused by this interfering quantity. The pixeldisplacement of the line centroid from one image line to the next servesas the amplitude of the image interference. The line frequency permitsan assignment of time and frequency for the correlation in the case ofthe active compensation signal application of the feed forward signal.If the external sensor is read-in in parallel with the determination ofthis pixel displacement at the beginning of each line, a time-parallelor simultaneous detection of the image interference and of theinterfering influence that causes this interference can be performed. Inprinciple, assuming sufficient coherence, it is thus possible todirectly calculate the transfer function to be set at the filter 5 inorder to essentially compensate for the image interference. In analternative embodiment, fundamental assumptions are made concerning thepoles and zeros of the transfer function of the filter, and theindividual parameters of the variably configured functions are optimizediteratively.

An method for determining the centroid displacement of successive linesis briefly outlined below. On the basis of the sampling theorem, it ispossible to compensate for interference frequencies, which are less thanhalf the sampling frequency. Furthermore, the method presupposes thatindividual objects within the image are very much larger than the linespacing and that centroid displacements perpendicular to the scanningdirection in the image are small in comparison with centroiddisplacements parallel to the line direction. Moreover, it is assumedthat the difference in the intensity å_(n) (t) of neighbouring lines issmall, and the intensity f_(n+1) of the line n+1 can be written asfollows:i. f _(n+)1(t)=f _(n)(t)+å_(n)(t).

If this system is then interfered with, assuming that the interferencecauses a temporal displacement Ä_(n) of the pixels within the line, thedisturbed intensity d_(n), (t) is given by:d _(n+1)(t)=t _(n+1)(t+Ä _(n+1))=f _(n)(t=Ä _(n+1))+å_(n)(t+Ä _(n+1))and d _(n+1)(t)=d _(n)(t+Ä _(n+1)−Ä_(n))+å_(n)(t+Ä _(n+)).

Using a non-causal Wiener filter, it is possible to calculate a a pulseas a function of the line displacements Ä_(n+1) and Ä_(n):å(t+Ä _(n+1))≈FFT ⁻¹ {D* _(n+1)ù)/|D _(n)(ù)|²+ä²å},where D_(n), (ù) is the Fourier transform of the disturbed intensityd_(n)(t). This a function depends on the difference between the centroiddisplacement of neighbouring lines. Consequently, the centroiddisplacement within the lines of an image can be calculated recursively,since, as explained above (Ä_(n+1)−Ä_(n)) is known as a result of theimage analysis.

For the driving of the deflection unit of the microscope, a signal whichis proportional to the correlation function of the measured interferingquantity and the calculated centroid displacements in the individuallines is generated using a vector correlation. This correlation iscarried out in the digital filter, a second signal, which is dependenton the temporal displacement calculated, being applied to thecalibration input of the filter.

A further embodiment of the invention (FIG. 8) is suitable for examplefor transmission electron microscopes (TEM) or light microscopes orrelated types of apparatuses which use a camera system 20 to display theobject. In the embodiment described below, the apparatus illustrated inthe block diagram in FIG. 1 c corresponds to the optical microscope 18illustrated in FIG. 8. The external sensor 4 is designed as a multi-axisvibration sensor whose signal is passed via an adjustable filter 5 andan amplifier 6 to a control element, which, in the present embodiment,is assigned directly to the imaging processing device 21 and has aneffect on the image in the latter. In FIG. 8, the filter, the amplifierand the control element are not explicitly shown but rather arecontained integrally in the image processing device 21. According to theinvention, then, in this apparatus a compensation signal is not appliedto an actuator which influences the imaging, rather, instead of this,the image display is influenced directly. The camera system comprises aCCD camera 19 with a monitor, an image frequency of 25 Hz being workedwith. The image processing device 21 is set up for storing successiveimages. By means of image analysis, the displacement of the imagecentroid of successive images in two mutually orthogonal directions iscalculated and used to set the transfer function of the digital filter 5in a similar manner to that in the embodiment described above. Anillustrative representation of this displacement of the centroid ofsuccessive images is shown in FIGS. 6 and 7. The curve 17 in FIG. 7shows the profile of the coordinate x of the centroid with time. Thedifferences between two scanning points, for example t₀ and t₁, thuscorrespond to the image refresh frequency. A further embodiment, incomparison with the embodiment described above, enables instances ofinterference to be corrected by the compensation signal application evenat frequencies which are greater than the image refresh frequency of 25Hz. For this purpose, the transfer function, which is defined by thepoints of resonance in the mechanical construction of the microscope, isimplemented as the filter 5. In this way, a base vibration X generates arelative movement Äx at the microscope. The general transfer function isthus completely determined by the actual sensitivity Äx/X, the resonantfrequency f_(R) and by the parameter Q, which defines the asymptoticdecline of Äx/X at high frequencies. By determining three points on thecurve below the resonant frequency f_(R) as well, it is thus possible toinfer the entire function and use it in the feed forward control byapplication of a compensation signal also for interference frequencieswhich are greater than the image refresh frequency.

In contrast to the embodiments described heretofore, according to theinvention it is possible, moreover, to use the image information not ina feed forward arrangement but in a traditional feedback arrangement forthe compensation of image interference. This is illustratedschematically in the block diagram 1 d. The sensor whose signal is fedforwards is omitted, and instead of this the centroid displacementsdetermined in the x- and/or y-axis from the image analysis are fed backinto a suitable control element, in this case a device for displacingthe sample, after passing through an adjustable transfer function.

In further embodiments (not illustrated in any detail here) of theinvention, the apparatus may be a force microscope, a surface roughnessmeasuring instrument, an optical scanning microscope or a lithographyinstallation.

Depending on the embodiment, in the case of electron microscopes, thedriven actuators and control elements comprise the already describedelectron beam deflection devices and/or control elements in the imageprocessing device, and in the case of optically operating apparatuses,the actuators comprise, depending on the embodiment, devices fordeflecting the light and/or devices for deflecting the sample and/orcontrol elements in the image processing device. A control element inthe image processing device in this case designates, by way of example,the influence on a parameter which has effects on the calculation of theimage. Moreover, use is made of further actuators which are sensitive tovibrations, and also force actuators electrodynamic linear drives) anddistance drives (piezotranslators).

1. At least one of an imaging and raster-mode scanning apparatuscomprising a sample holder for holding a sample object, one meansselected from the group comprising means for generating an electronbeam, means for generating a light beam, and means for determining aforce onto said sample object actuator means for moving one of saidelectron beam, light beam or force determining means relative to a saidsample object so as to form a scanner, means for forming an image whenthe position of one of said electron beam, light beam or forcedetermining means is moved relatively to said sample object andoptionally to a predetermined reference object so as to produce imagesignals, an image processor for processing said image signals, an imagedisplay device, an electrical filter having a signal input and acalibration input, at least one sensor that provides a first signaldependent on ambient influence that might interfere with proper imagingand lead to image degradation, said electrical filter having a settabletransfer characteristic that can be set by applying a second signal tosaid calibration input of said electrical filter so as to calibrate saidapparatus, wherein said ambient influences detected by said sensor arecompensated to a certain extent, and wherein said first signal dependenton the ambient influences passes through said electrical filter and iscombined with driving signals for said actuator means of the apparatusto compensate said ambient influences that might interfere with properimaging.
 2. The apparatus according to claim 1, wherein the at least onesensor is adapted to detect at least one physical quantity outside theapparatus, and to output the first signal that depends on the ambientinfluences at the location of the at least one sensor.
 3. The apparatusaccording to claim 2, wherein the at least one sensor comprises at leastone pick-up for electromagnetic fields, air vibrations and groundvibrations.
 4. The apparatus according to claim 1, wherein said signalinput of the electrical filter is connected to an output of said imageprocessor.
 5. The apparatus according to claim 1, further comprising acalibrator that mutually calibrates the filter.
 6. The apparatusaccording to claim 1, said electrical filter is a digital filter.
 7. Theapparatus according to claim 1, wherein an output of the image processoris connected to said calibration input of the electrical filter.
 8. Theapparatus according to claim 1, wherein the second signal varies as afunction of said relative position of said electron or light beam tosaid object.
 9. The apparatus according to claim 1, wherein theapparatus operates in a calibration mode and subsequently operates in animage mode, whereby, in the calibration mode, a first image signal ofsaid reference object is provided in said image processor, a secondimage signal of said reference object is provided under ambientinfluences, then the first image signal and the second image signal arecompared in the image processor resulting in an error signal which issaid second signal for calibrating said electrical filter by settingsaid transfer characteristic thereof, and wherein by calibration of theelectrical filter, ambient influences that degrade the image signals arecompensated to a certain extent.
 10. The apparatus according to claim 9,wherein in the calibration mode: said first image signal of thereference object being present in said image processors, the apparatusscans, under ambient influences, a selected section of the referenceobject so as to obtain said second reference image signal, the imageprocessor compares said first reference image signal with said secondreference image signal, so as to form said error signal from anydifference resulting from said comparison, and wherein the apparatusstores, in a memory, data for generating the second signal for settingthe transfer characteristics of the electrical filter for the imagemode.
 11. The apparatus according to claim 10, wherein in the imagemode: the apparatus scans the sample object to be imaged, and takingsaid data stored during the calibration mode as a basis, generates thesecond signal for defining the transfer characteristics of theelectrical filter.
 12. The apparatus according to claim 2, wherein theapparatus is set up for automatically calibrating the electrical filterduring an image mode.
 13. The apparatus according to claim 12, whereinsaid image forming means is adapted to scan said sample object to formsuccessive image lines which define line centroids, or image centroids,and said image processor is set up for determining a temporaldisplacement of said line centroids of successive image lines across theimage and outputs to the electrical filter, the second signal as afunction of this temporal displacement.
 14. The apparatus according toclaim 13, wherein the image processor is set up for determining atemporal displacement of said image centroids of successive imagesscanned by the image forming means and outputs the second signal as afunction of this temporal displacement, as determined, to the electricalfilter.
 15. The apparatus according to claim 12, wherein the electricalfilter is set up for carrying out a cross-correlation of the firstsignal and of the second signal.
 16. The apparatus according to claim14, in the form of a light microscope or a transmission electronmicroscope also comprising means for analyzing temporal displacement insaid image signals, the first signal also being determined from thetemporal displacement that is determined in said image signals.
 17. Theapparatus according to claim 1, wherein the apparatus is set up forreducing or compensating for the image degradation in two mutuallyorthogonal directions.
 18. The apparatus according to claim 1, whereinthe apparatus comprises one of a scanning electron microscope, a forcemicroscope, a surface roughness measuring instrument, an opticalscanning microscope, a light microscope, a transmission electronmicroscope or a lithography installation.
 19. The apparatus according toclaim 18, wherein in the case of the electron microscope, said actuatormeans comprises at least one of a deflector for deflecting electron beamand a displacer that displaces said sample object.
 20. The apparatusaccording to claim 18, wherein in the case of the optical scanningmicroscope, said actuator means comprises a deflector device fordeflecting said light beam or a displacer that displaces said sampleobject.
 21. The apparatus according to claim 7, wherein the apparatus isdesigned for operation in a calibration mode and for subsequentoperation in an image mode, whereby, in the calibration mode a firstimage signal of said reference object is provided, and under ambientinfluences, a second image signal of said reference object is provided,then the first image signal and the second image signal are compared inthe image processor, wherein the comparison results in a differencerepresenting an error signal being assigned to the second signal forsetting the transfer characteristic of said electrical filter, wherebyby calibration of the electrical filter ambient influences which degradethe image signals are compensated to a certain extent.
 22. The apparatusaccording to claim 7, wherein the apparatus is set up for automaticallycalibrating the electrical filter.
 23. The apparatus according to claim7, in the form of a light microscope or a transmission electronmicroscope also comprising means for analyzing temporal displacement insaid image signals, the first signal also being determined from saidtemporal displacement in said image signals.
 24. The apparatus accordingto claim 4, for operation in a calibration mode and subsequentlyoperable in an image mode, whereby, in the calibration mode, a firstimage signal of said reference object is provided, and under ambientinfluences, a second image signal of said reference object is provided,then the first image signal and the second image signal are compared inthe image processor, wherein the comparison results in a differencerepresenting an error signal being assigned to the second signal forsetting the transfer characteristic of said electrical filter as toreduce ambient influences which might degrade imaging.
 25. A lightmicroscope comprising a sample holder for holding a sample object, acamera system for forming an image of said sample object, actuator meansfor moving said camera system relative to said sample object, means forconverting said image of said sample object and optionally of apredetermined reference object into image signals, an image processorfor processing said image signals, an image display device, a digitalelectrical filter having a signal input and a calibration input, whereinsaid image processor, based on analysis of successive image signals,provides a first signal, to be supplied to said signal input of saiddigital electrical filter, wherein said image converting means and saidimage processor cooperate to provide a second signal to be supplied tosaid calibration input of said digital electrical filter, saidelectrical filter having a settable transfer characteristic that can beset by applying said second signal to said calibration input of theelectrical filter to effect the apparatus into a calibrated state,wherein said ambient influences are compensated to a certain extent, andwherein said first signal passes through said electrical filter and iscombined with driving signals for said actuator means of the apparatusto compensate said ambient influences.
 26. A method for operating animaging or raster-mode scanning apparatus for compensating ambientinfluences that may degrade the imaging of a sample object, theapparatus including an internal actuator or internal control element andan image processor, comprising the steps of: providing a driving signaland driving said actuator or control element to produce an image of thesample object, providing a first signal dependent on the ambientinfluences, supplying said first signal to a signal input of anelectrical filter having a settable transfer characteristic which can beset by applying a second signal to a calibration input of the electricalfilter, and passing the first signal directly through said electricalfilter, providing an output signal of the electrical filter, combiningsaid driving signal for said internal control element of the apparatuswith said output signal of said electrical filter, which has an effecton the imaging of said image processor, effecting the apparatus into acalibrated state, by applying said second signal to the calibrationinput of the electrical filter for setting the transfer characteristic,such that any image degradation from ambient influences is compensatedto a certain extent.
 27. The method according to claim 26, wherein thecalibration of the apparatus is carried out by manual setting of theelectrical filter.
 28. The method according to claim 26, wherein saidinternal control element is a member of said image processor foreffecting the compensation of the image degradation.
 29. The methodaccording to claim 26, wherein said internal actuator is a means formoving an electron beam relatively to a sample object so as to form ascanner and the compensation of the image degradation is carried out atleast partially by driving said internal actuator.
 30. The methodaccording to claim 26, also including: providing a sensor which isarranged outside said imaging or raster-mode scanning apparatus and isfor detecting ambient influences that degrade the imaging, and drivessaid first signal input of said electrical filter, providing apredetermined reference object and a first image signal of saidreference object, wherein the apparatus is operated in a calibrationmode by applying said second signal to said calibration input of saidelectrical filter which is produced by imaging of said reference objectunder ambient influences to obtain a second image signal and comparingthe second image signal of the reference object with said first image ofthe reference object, and wherein the apparatus is subsequently operatedin the image mode.
 31. The method according to claim 30, wherein thecalibration mode comprises at least the following steps: acquiring animage of a selected section of the predetermined reference object byscanning the reference object avoiding degradations so as to producesaid first image signal of the selected section; determining the firstsignal which depends on any ambient influence at the location of thesensor, which is arranged outside said imaging or raster-mode scanningapparatus; applying the first signal to the signal input of saidelectrical filter; acquiring an image of said selected section of thepredetermined reference object by scanning of the reference object underambient influence so as to produce said second image signal of theselected section; comparing said second image signal of the selectedsection of the reference object under ambient influences with said firstimage signal of the reference object so as to form an error signal whichis a difference between said first image signal and said second imagesignal; applying said second signal, derived from said error signal, tothe calibration input of said electrical filter for setting the transfercharacteristic of the electrical filter; applying the signal of theelectrical filter to the signal input of a regulating amplifier;applying the output signal of said regulating amplifier to said internalactuator which is for scanning said reference object by deflecting anelectron or light beam by moving a holder for said reference objectrelative to said beam, said deflecting of said beam or said moving ofsaid holder being influenced so as to correct imaging; repeating theiterations of the steps of comparing said second image signal and saidfirst image signal so as to modify said characteristic of saidelectrical filter for minimizing said error signal and storing datadetermined by iterative calibration for providing the transfercharacteristic of the electrical filter for said image mode.
 32. Themethod according to claim 30, wherein the image mode comprises thefollowing steps: acquiring an image signal of said sample object byscanning, with said settable transfer characteristic of said electricalfilter being fixed in said calibration mode, passing said output signalof said electrical filter through a regulating amplifier, and supplyingsaid amplifier output signal to said internal actuator to controlelement.
 33. The method according to claim 26, wherein said imageprocessor makes an image analysis of an image of a sample object or areference object acquired by said imaging or raster-mode scanningapparatus and produces a setting signal dependent on such image analysiswhich is applied as said second signal to said calibration input of saidelectrical filter.
 34. The method according to claim 33, wherein theimage analysis comprises a recursive determination of a temporaldisplacement of line centroids of successive image lines within theimage of said reference object, and whereby said second signal iscalculated from said temporal displacement.
 35. The method according toclaim 33, wherein successively images of said reference object aretaken, wherein the image analysis comprises a recursive determination ofa temporal displacement of image centroids of said successive image, andwherein said second signal is calculated from said temporaldisplacement.
 36. The method according to claim 34, wherein essentiallya cross-correlation of the first signal with the second signal iscarried out and an output signal of the electrical filter which isdependent on the cross-correlation between the first signal and thesecond signal is supplied to said actuator or control element.
 37. Themethod according to claim 26, comprising the steps of feeding said imageprocessor with an image signal from an image acquirer; applying a signaldependent on the result of said analyzing step as said first signal tosaid signal input of the electrical filter; applying a signal dependenton the result of the analyzing step as the second signal to saidcalibration input of the electrical filter; and applying the output ofthe electrical filter via a regulating amplifier to said internalactuator or said internal control element so as to reduce imagingdegradation.
 38. The method according to claim 37, wherein, insuccessive time periods, successive images of said sample object or of areference object are produced, successive image lines within anysuccessive image and the centroids thereof or image centroids ofsuccessive images are determined, and wherein said analyzing stepcomprises a recursive determination of any displacement of said linecentroids of said successive image lines within the image or a recursivedetermination of any displacement of said image centroids of successiveimages.
 39. The method according to claim 26, wherein control elementsacting in two mutually orthogonal directions are provided forcompensating any image degradation.
 40. An apparatus for compensatingfor ambient influences in imaging or raster-mode scanning apparatusesthe may degrade the imaging with an image acquisition and an imageprocessing device producing an image of a sample object or a referenceobject, comprising a calibratable digital electrical filter with asignal input and a calibration input; a regulating amplifier which iselectrically connected downstream of the electrical filer, an internalcontrol element controlled by the regulating amplifier; wherein a secondsignal is applied to the calibration input of the electrical filter tocalibrate the electrical filter; and wherein the internal controlelement has an effect on said image produced by said image acquisitionand image processing device, whereby in the calibrated state of theelectrical filter, the image degradation is compensated to a certainextent.
 41. The apparatus according to claim 40, further comprising atleast one sensor for detecting at least one physical quantity outsidethe imaging or raster-mode scanning apparatus, this sensor outputtingthe first signal which is dependent on the ambient influences at thelocation of the sensor.